1. Field of the Invention
The present invention relates to a method of forming a semiconductor layer, and more particularly, to a method of crystallizing amorphous silicon for a flat panel display device and a thin film transistor fabricated using a method of crystallizing amorphous silicon.
2. Discussion of the Related Art
Presently, flat panel display (FPD) devices are being developed having high portability and low power consumption. Among various types of FPD devices, liquid crystal display (LCD) devices are commonly used as monitors for notebook and desktop computers because of their ability to display high resolution images, wide ranges of different colors, and moving images.
In general, an LCD device includes a color filter substrate and an array substrate separated from each other by a liquid crystal layer, wherein the color filter substrate and the array substrate include a common electrode and a pixel electrode, respectively. When a voltage is supplied to the common electrode and the pixel electrode, an electric field is generated that affects orientation of liquid crystal molecules of the liquid crystal layer due to optical anisotropy within the liquid crystal layer. Consequently, light transmittance characteristics of the liquid crystal layer become modulated and images are displayed by the LCD device.
The array substrate includes thin film transistors (TFTs) that function as switching elements. Since amorphous silicon may be formed on low-cost glass substrates at low temperatures, amorphous silicon is commonly used for active layers in the TFTs of an LCD device. In addition, since a driving circuit is required to drive the amorphous TFT (a-TFT), the driving circuit includes a plurality of complementary metal-oxide-semiconductor (CMOS) elements having single crystalline silicon layers. Large-scale integration (LSI) circuits using single crystalline silicon layers are commonly connected to array substrates using amorphous silicon layers via connection systems, such as tape automated bonding (TAB). However, since the costs of fabricating the driving circuits are relatively high, costs of LCD devices using the driving circuits is also relatively high.
Currently, LCD devices are being developed to include TFTs using polycrystalline silicon as the active layer (poly-TFTs). Accordingly, driving circuits of the LCD devices incorporating poly-TFTs can be formed on the same substrate as the TFTs formed within pixel regions. Thus, additional processes for connecting the TFTs of the pixel regions with the driving circuits may be eliminated, thereby material costs for the driving circuits are reduced and the process of fabricating the LCD devices is simplified. In addition, since polycrystalline silicon has a field effect mobility greater than a field effect mobility of amorphous silicon, the LCD device incorporating the poly-TFTs has a faster response time and increased resistance to adverse effects due to heat and light.
Polycrystalline silicon may be deposited directly, or may be formed by crystallizing amorphous silicon deposited through a plasma enhanced chemical vapor deposition (PECVD) method or by a low pressure chemical vapor deposition (LPCVD) method. Methods of crystallizing amorphous silicon may be classified into a solid phase crystallization (SPC) method, a metal induced crystallization (MIC) method, an excimer laser annealing (ELA) method, and a sequential lateral solidification (SLS) method. Among these various different methods of crystallizing amorphous silicon, an ELA method using ultraviolet (UV) light produced by an excimer laser is commonly used. In the ELA method, since a layer of amorphous silicon is annealed for a short time period, a substrate is not deteriorated even under a melting temperature of silicon. Accordingly, a polycrystalline silicon layer of excellent crystallinity is obtained by annealing an amorphous silicon layer with an excimer laser.
An ELA method includes melting a layer of amorphous silicon and instantaneous solidification when a laser beam of an excimer laser is irradiated onto the melted amorphous silicon. FIG. 1 is a graph showing a relationship between grain size and laser energy density from an excimer laser annealing method according to the related art. As shown in FIG. 1, an amorphous silicon layer having a thickness of about 500 Å shows that melting depth and crystallinity are determined by the energy density of the laser beam.
In FIG. 1, as an energy density of a laser beam increases, a melting depth from a top surface to an interface between liquid and solid states of the amorphous silicon increases and an amount of melted silicon also increases, wherein the amorphous silicon layer completely melts over a critical energy density “EC.” The melting amorphous silicon layer is re-crystallized and converted into a polycrystalline silicon layer. Accordingly, crystallinity of the polycrystalline silicon layer depends on a state of melting the amorphous silicon layer.
A range of an energy density of the laser beam irradiated onto the amorphous silicon layer may be classified into three different regimes according to a state of melting the amorphous silicon layer: a partial melting (PM) regime; a nearly complete melting (NCM) regime; and a complete melting (CM) regime. In the PM regime, only an upper portion of the amorphous silicon layer melts. Accordingly, while the melting upper portion is re-crystallized, a grain vertically grows using the lower portion of the amorphous silicon layer as a seed. As a result, although grain size variation according to an energy density is small, grain size is also small within the PM regime. In the NCM regime, most of the amorphous silicon layer melts, to an interface of the amorphous silicon and the substrate, and a polycrystalline silicon layer is produced having a grain size of about 1000 Å to about 6000 Å. Specifically, as the energy density approaches the critical energy density “EC,” grain size sharply increases. In the CM regime, all of the amorphous silicon layer melts and grain size is small due to homogeneous nucleation. Since a grain size of the polycrystalline silicon layer is largest within the NCM regime, energy density in the NCM regime is selected to crystallize the amorphous silicon layer.
FIGS. 2A to 2C are schematic cross sectional views of an amorphous silicon crystallization process according to the related art. In FIG. 2A, most of a semiconductor layer 13 of amorphous silicon melts to an interface 14 between the semiconductor layer 13 and a substrate 10 immediately after a laser beam is irradiated within a completely melting regime. For example, the semiconductor layer 13 may include a melting portion 16 and non-melting portions 15 of amorphous silicon having first, second, third, and fourth sizes g1, g2, g3, and g4, and the non-melting portions 15. may be separated by first, second, and third distances d1, d2, and to d3.
In FIG. 2B, grains 17a grow using the non-melting portions 15 as seeds while the melting portion 16 is solidified.
In FIG. 2C, the grains 17b and 17c continue growing, thereby solidifying the whole semiconductor layer 13 to have final grains 17 of polycrystalline silicon. Since the positions of the non-melting portions 15 are randomly disposed, the final grains are randomly disposed and are not controllable.
However, within the NCM regime, grain size variation is large due to an energy density of the laser beam. Moreover, when a laser beam corresponding to the CM regime is irradiated, grain size is abruptly reduced. Thus, since grain size greatly varies even with small changes of the energy density, the ELA process has a narrow process window for achieving optimum results, wherein the process window is a process error margin where deterioration does not occur. As the process window becomes narrower, production yield decreases and production costs increase.
An excimer laser using source gases, such as xenon chloride (XeCl), having a wavelength of 308 nm does not irradiate a laser beam having an exact energy density. For example, when an energy density of 260 mJ/cm2 is set for the laser beam, the excimer laser cannot irradiate a laser beam having an exact energy density of 260 mJ/cm2 due to an inherent error of the excimer laser apparatus. Accordingly, grain size and distances between the grains are not uniform in the layer of polycrystalline silicon. Consequently, as shown in FIGS. 2A-2C, since positions and grain sizes g1, g2, g3, and g4 of the non-melting portions 15 of amorphous silicon and distances d1, d2, and d3 between the non-melting portions 15 are not controllable due to the narrow process window, the final grain sizes of the polycrystalline layer 13 are not uniform. Thus, the non-uniformity of the grain sizes deteriorates operational characteristics of a TFT using the polycrystalline layer 13.